Wireless communication systems are generally composed of one or more local central sites, herein termed base stations, through which wireless transmitter/receivers gain access to a larger information network. The base stations service a local area wherein a number of wireless users, fixed or mobile, are located. The function of the base station is to relay messages to and from users all over the network. In cellular mobile systems, for example, this task is performed by relaying messages to and receiving signals from a Mobile Telephone Switching Office (MTSO). A wireless user establishes a two-way (full-duplex) communication link with one or more other users also having some access to the network by first requesting access to the network through the local base station. This communication is accomplished in cellular mobile communications and wireless local area computer networks (LANs), for example, by suitably modulating electromagnetic waves.
Conventional wireless communications systems requires that users transmit signals in different frequency channels, use different coding schemes in the same frequency channels, or be transmitted in non-overlapping time intervals for the signals to be correctly received. One aspect of the present invention describes a method and apparatus for separating multiple messages in the same frequency, code, or time channel using the fact that they are in different spatial channels. Hereinafter, the term channel will be used to denote any of the conventional channels (frequency, time, code) or any combination thereof. The term spatial channel refers to the new concept unique to this the present invention.
Wireless communication is becoming an increasingly common form of communication (D. Goodman, "Trends in Cellular and Cordless Communications," IEEE Communications Magazine, June 1991), and the demand for such service continues to grow. Examples include cellular mobile communication networks, wireless local area computer networks, wireless telephone networks, cordless telephones, satellite communication networks, wireless cable TV, multi-user paging systems, high-frequency (HF) modems, and more. Current implementations of these communication systems are all confined to limited frequency bands of operation either by practical considerations or, as is more often the case, by government regulation. As the capacity of these systems has been reached, demand for more service must be met by allocating more frequency spectrum to the particular application along with attempts to utilize the allocated spectrum more efficiently. In light of the basic physical principle that transmission of information requires bandwidth, the fundamental limitations of a finite amount of practically usable spectrum present a substantial barrier to meeting an exponentially increasing demand for wireless information transmission. Since, as has been demonstrated over the last decade, the amount of practically usable frequency spectrum can not keep pace with the demand, there is a critical need for new technology for increasing the ability of such systems to transfer information (D. Goodman, op. cit., G. Calhoun, Digital Cellular Radio, Artech House 1988). This invention directly addresses this need and is compatible with current as well as future modulation schemes and standards (D. Goodman, "Second Generation Wireless Information Networks," IEEE Trans. on Veh. Tech., Vol. 40, No. 2, May 1991).
In conventional wireless communications systems, a base station serves many channels by means of different multiple access schemes, the most common being Frequency-Division Multiple Access (FDMA), Time-Division Multiple Access (TDMA), and more recently Code-Division Multiple Access (CDMA). All current systems employ FDMA wherein the available frequency bandwidth is sliced into multiple frequency channels and signals are transmitted simultaneously, with a maximum of one per channel at any given time. All wireless systems also currently employ TDMA, a technique wherein multiple users share a common frequency channel by doing so at different times, in that when a user no longer requires the channel assigned to it, the channel is reassigned to another user.
In the conventional wireless communications systems, TDMA is also being exploited on a more detailed level. Analog data such as voice is digitized, compressed, then sent in bursts over an assigned frequency channel in assigned time slots. By interleaving multiple users in the available time slots, increases in the capacity (i.e., number of simultaneous users) of the system can be achieved. This requires substantial modifications to the base station receiver hardware as well as the mobile units themselves, however, since the current analog units are not capable of exploiting this technology. Consequently, a dual-mode standard, supporting both the new digital and the old analog transmission schemes, has had to be adopted.
CDMA allows multiple users to share a common frequency channel by using coded modulation schemes. The technology involves preprocessing the signal to be transmitted by digitizing it, modulating a wideband coded pulse train, and transmitting the modulated coded signal in the assigned channel. Multiple users are given distinct codes which decoders in the receivers are programmed to detect. If properly designed, the number of simultaneous users of such a system can be increased over conventional wireless communications systems. While theoretically sound, however, the technology has yet to be proven. There are substantial practical problems with the scheme, the most important being a stringent requirement for accurate and rapid power control of the wireless transmitters. Such problems may vitiate the usefulness of CDMA in wireless communication networks. Should CDMA survive, however, the SDMA concept described herein can be applied directly to further increase capacity and system performance.
The aforementioned techniques represent various attempts to more efficiently pack an increasing number of signals into fixed-width frequency channels. These techniques do not exploit the spatial dimension when establishing channels. This invention describes how, in addition to traditional schemes, the spatial dimension can be exploited to significantly increase the quality of the communication links, reduce the required amount of transmitted power, and most importantly increase the number of channels that a base station can serve without allocation of more frequency channels. This technique is hereafter referred to as Spatial-Division Multiple Access (SDMA).
In conventional wireless communications systems, exploitation of the spatial dimension is limited to what is referred to as spatial diversity and sectorization. In spatial diversity, most commonly associated with mobile systems, two antennas are employed on reception only, and the one with the strongest signal in the bandwidth of interest is chosen for further processing, or some method for combining the two outputs is applied (P. Balaban and J. Salz, "Dual Diversity Combining and Equalization in Digital Cellular Mobile Radio", IEEE Trans. on Veh. Tech., Vo. 40, No. 2, May 1991). Though this leads to a minor improvement in the quality of the received signal, there is no increase in system capacity.
To increase the capacity of cellular systems, service providers have been installing more cell sites, reducing the area covered by each site so that more users can access the system. The idea is that signals far enough away will not interfere with local sources since power dissipates quite rapidly in space the further from the transmitter the receiver is located. This straightforward approach to increasing capacity is, however, quite costly as the amount of cell site hardware required is proportional to the number of cell sites, which in turn is inversely proportional to the square of the factor by which the effective radius of each cell is decreased. In fact, the current economics of the situation dictate that service providers bid for precious frequency spectrum before even considering installation of new cell sites (G. Calhoun, Digital Cellular Radio, Artech House 1988). Furthermore, this strategy also greatly exacerbates the hand-off problem as discussed further on since users enter and leave cells more often when the cells are smaller.
Sectorization is similar in spirit and is another technique for increasing capacity by essentially making the local areas served by each cell smaller, thus adding more cells to the network. This is accomplished at a common location by employing directive antennas, i.e., receiving antennas at the cell site which receive mobile transmissions is a particular sector only. Patents related to this basic cellular concept have been issued to Motorola in 1977 (V. Graziano, "Antenna Array for a Cellular RF Communications System," U.S. Pat. No. 4,128,740, 13/1977, U.S. Cl. 179-2 EB), Harris Corporation in 1985 (M. Barnes, "Cellular Mobile Telephone System and Method," U.S. Pat. No. 4,829,554, 55/1985, U.S. Cl. 379-58), NEC Corporation in 1986 (M. Makino, "Mobile Radio Communications System," U.S. Pat. No. 4,575,582, C.I.P. U.S. Pat. No. 4,796,291, 3/1986, U.S. Cl. 358-58), and Sony Corporation (T. Kunihiro, "Cordless Telephone," U.S. Pat. No. 4,965,849, 9/1989, U.S. Cl. 455-34) to name just a few. With recent developments in digital technology making digital transmission and reception of information economically feasible, there have been a significant number of patents in this area as well including S. Hattori, et al., "Mobile Communication System," U.S. Pat. No. 4,947,452, 10/1989, U.S. Cl. 455-33; S. Hattori, et al., "Mobile Communication System," U.S. Pat. No. 4,955,082, 1/1989, U.S. Cl. 455-33; T. Shimizu, et al., "High Throughput Communication Method and System for a Digital Mobile Station When Crossing a Zone Boundary During a Session," U.S. Pat. No. 4,989,204, 12/1989, U.S. Cl. 370-94.1; T. Freeburg, et al., "Cellular Data Telephone System and Cellular Data Telephone Therefor," U.S. Pat. No. 4,837,800, 13/1988, U.S. Cl. 379-59; and R. Mahany, "Mobile Radio Data Communication System and Method," U.S. Pat. No. 4,910,794, 6/1988, U.S. Cl. 455-67. Though sectorization increases capacity, it has limited potential for meeting future demand and is fundamentally limited by the basic physical principles that do not permit the design of exceedingly small sectors without exceedingly large antennas. Furthermore, since sectorization is simply another method for increasing the number of cells, the hand-off problem which is discussed in detail further on, is exacerbated.
In the aforementioned conventional systems, it is assumed that there is only one mobile unit at a time transmitting in a given cell on a given frequency. Other transmitters actively transmitting in the same frequency channel at the same time are considered to be cochannel interference, a situation which current systems attempt to prevent since it leads to significant performance degradation. Cochannel interference, in fact, is a major factor in determining how often (spatially) frequency channels can be reused, i.e., assigned to different cells (W. Lee, Mobile Cellular Telecommunication Systems, McGraw-Hill, 1989). The cochannel interference problem pervades all wireless communication systems, not just cellular mobile communications, and attempts to solve it in current systems have all been formulated on the premise that the cochannel signals represent disturbances to be eliminated an that only one antenna/receiver output is available for the task.
Conventional systems in which interference suppression is performed using adaptive filters in the time-domain and the output of a single antenna includes F. Gutleber, "Interference Cancelling System for a Mobile Subscriber Access Communications System," U.S. Pat. No. 4,434,505, 14/1982, U.S. Cl. 455-50; and Y. Shimura, "Base Station Capable of Monitoring Occurrence of Interference on Every Transmission," U.S. Pat. No. 4,837,801, 8/1987, U.S. Cl. 379-61. These techniques are based on an assumption of statistical stationarity, i.e., that the channel characteristics do not change very fast. In the mobile communications environment where deep Rayleigh fading (40 dB) at rates up to 200 Hz is a dominant factor, the stationarity assumption is known to be invalid, and the performance of these conventional techniques is known to be quite susceptible to errors in the assumptions made. In particular, in the presence of multiple delayed copies of the same signal (i.e., specular multipath), these adaptive filters can null the desired signal.
Time-domain adaptive filter techniques have also been developed to improve channel quality for digital transmission in the presence of the aforementioned Rayleigh fading which causes intersymbol interference at the receiver. Examples of conventional techniques for addressing this type of interference include J. Proakis, "Adaptive Equalization for TDMA Digital Mobile Radio", IEEE Trans. on Veh. Tech., Vo. 40, No. 2, May 1991, and numerous other technical references in the open literature. Similar equalization techniques have been adopted in the current digital GSM system. The foregoing systems are completely compatible with this invention and can be incorporated in the demodulation step as is currently done in practice.
More recently, investigations have been undertaken into the possibility of combining the outputs of more than one antenna to improve signal quality by eliminating cochannel interference. In the context of wireless LANs and PBXs, a multi-channel adaptive equalization scheme has been described by J. Winter, "Wireless PBX/LAN System with Optimum Combining," U.S. Pat. No. 4,639,914, 9/1984, U.S. C. 370-110.1. This method relies on code assignment (CDMA) to a known number of transmitters and tight power control circuitry. It also requires time-division duplexing, i.e., transmission and reception at the base station and the wireless terminals must occur at different times on the same frequency. This requirement results from the fact that the information in the spatial dimension is not being fully exploited; source locations are not calculated. The aforementioned stationarity assumption is also critical to the technique and it is therefore not applicable to the mobile environment. Furthermore, it is modulation dependent and is designed solely for interoffice wireless LANs using digital transmission technology.
In the context of simply combating the cellular mobile problem of Rayleigh fading at the mobile receiver, a method of incorporating plural antennas is also described by P. Balaban and J. Salz, op. cit.. Herein as in similar well-known techniques, various assumptions concerning the temporal characteristics of the signal of interest and its relationship to the cochannel interfering signals are made and on the basis thereof, a time-varying filter is constructed with best possible reconstruction of the signal of interest as its sole purpose. The performance of this techniques is also known to be quite susceptible to errors in the assumptions made, specifically the stationary channel assumption. In fact, mobile unit implementation of this invention mitigates the Rayleigh fading problem to a large extent.
The undesirable characteristics of the aforementioned adaptive techniques are a consequence of the fact that only assumed time-domain properties of the received signals are being exploited, and that one of the signals present in the data is treated differently from the remaining signals, i.e., the cochannel interferers. It has been found this invention is that cochannel interferers simply represent a plurality of users attempting to access the system simultaneously on the same channel. Accordingly, one aspect of the present invention enables this situation to be managed regardless of the modulation type (analog or digital) and in the presence of multiple arrivals of the same signal (i.e., specular multipath) is a significant advantage over the above described conventional techniques.
Efficient exploitation of the spatial dimension to increase capacity requires the ability to separate a number of users simultaneously communicating on the same channel at the same time in the same local area (cell). As will be explained, one aspect of the present invention performs this separation by distinguishing the signals on the basis of their angle-of-arrival, information which is used to ascertain the location of the transmitters. The process of localization of the transmitter according to this aspect of the invention provides heretofore unexpected advantages over conventional techniques.
Localization of signals in space using data collected by an array of sensors has been accomplished in fields other than wireless communications. Such is the case, for example, in tracking of aircraft and other aerospace objects using phased-array radars. Examples of arrays with such structure include R. Roy, et at., "Methods for Estimating Signal Source Locations and Signal Parameters Using an Array of Signal Sensor Pairs," U.S. Pat. No. 4,750,147, 3/1985, U.S. Cl. 364-800, and R. Roy, et al., "Methods and Arrangements for Signal Reception and Parameter Estimation," U.S. Pat. No. 4,965,732, 7/1987, U.S. Cl. 364-460. The arrays used therein, however, are required to possess a special structure, i.e., sensors occur in pairs of identical elements. The present invention is not limited to the use of such specialized array structure.